IMO

Each language has its own personality, advantages, warts and inconsistencies. But each also seem to have an almost singular aspect that, if you really wish to be proficient in the language, you must know really, really well. Here's my list on my top 4:

• C: pointers
• Python: name space rules
• Java: Interfaces
• Ruby: Ducking

Of course, this is far from complete, and "expertise" in each topic doesn't imply expertise in the language itself, but without getting your head around these, I feel that you are limited by what you can do.